The invention relates to a catalyst for converting unburned hydrocarbons and for the reduction of nitric oxides alone and also with other combustible gases, such as ammonia or hydrogen.
The catalyst of the invention has been found through long experimentation and is based on the action of thorium oxide as a catalyst stabilizer in the production of catalytic layers. Catalysts based on noble metals, metal oxides and especially on metal compound oxides such as LaCoO.sub.3 require special stabilizers to provide a long service life and to maintain the catalyst activity, or as a protection against chemical changes caused by other layers such as wash coats and by their trace contaminants.
It has now been found that thorium oxide plays a surprising and decisive part as such a stabilizer and supplies an especially efficient catalyst substance in a special composition with the components lanthanum oxide and LaCoO.sub.3. The catalytic action is not achieved without the participation of thorium oxide in the catalyst complex.
A special field of application for such a catalyst is in the treatment of exhaust gases of Diesel engines, wherein the catalyst makes use of the unburned components, in particular soot and unburned hydrocarbons, as reduction agents for reducing nitric oxide. In this process, the unburned components are burned with the oxygen excess of the exhaust gas and the oxygen of the nitric oxides, and in this way are rendered harmless.
Previously, carbon black filters have been proposed which store the unburned substances in the wall area of ceramic bodies and subsequently burn these substances after the combustion temperature has been reached. In some cases, the combustion temperature was reduced due to the introduction of catalysts. These filters require a large volume in order to avoid an excessive loss of pressure in the exhaust system. Furthermore, residues of oil ash remain in the walls of the filter and are not removed in the combustion process. The residues then cause a continuous increase in the pressure loss of the filters. Finally, this leads to tearing of the filter walls, and the filter becomes unusable.
An invention according to German patent application No. P 33 40 682 proposed to avoid these disadvantages by placing an exhaust gas catalyst in the stream of exhaust gas of a Diesel engine, with the catalyst being expected to effect the combustion of the unburned components. However, with this invention, special problems are created by the considerable amount of carbon black or soot accumulating in the starting phase, because the reaction temperature of the catalyst is not yet reached in the starting phase.
Accordingly, it is an object of the present invention to provide a catalyst for reacting unburned hydrocarbons and reducing nitric oxides in diesel engine exhaust gases.
It is also an object of the present invention to provide an apparatus, employing the catalyst of the invention, for treating diesel engine exhaust.
It is another object of the invention to provide such an apparatus which does not permit any increase in the pressure loss while the engine is in operation.
It is still another object of the invention to provide a process for manufacturing the catalyst.
In particular, it is the intention of the apparatus of the present invention to combine and make use of the advantages of both of the earlier above-described systems, permitting both the storage of carbon black or soot during the starting phase of the diesel engine and a combustion of the carbon black in the load phase with the reduction of nitric oxide without the creation of any excessive pressure loss.